1. Field of the Invention
The present invention relates to a profile creation method and a profile creation apparatus, as well as an image processing apparatus and an image processing system that are for performing color conversion using profiles, and more particularly to color processing for performing gamut mapping that converts the color gamut of an input device to the color gamut of an output device.
2. Description of the Related Art
Wide spread use of digital devices such as digital cameras and image scanners in recent years has made it easy to obtain digital images. This in turn has provided increasing opportunities to display obtained digital images on a monitor, confirm and process the images, and output the images with an inkjet printer or the like. However, various imagers such as monitors and printers generally have different color gamuts (the range of colors that can be represented). For example, there are colors that can be represented by monitors but not by printers, and vice versa. Accordingly, the colors that can be represented by monitors but not by printers need to be replaced by colors that can be represented by printers (color gamut compression). Likewise, when the color gamut of images input by a scanner or digital camera is different from the color gamut output by a monitor or printer, the color gamut of input image data needs to be converted to the color gamut of output image data.
The color gamut compression is carried out using a color profile with which color management is performed. Generally, for example, color profiles proposed by the International Color Consortium (ICC), which are widely used as industry standard, are well-known, but color gamut compression is also carried out with proprietary color profiles of printer companies (color management). Also, the recent emergence of printers with a built-in spectrophotometer (or colorimeter) has increased demand for a technique with which the printer user can independently and automatically generate color profiles for a variety of additional paper types.
The color gamut compression used in the color profile is generally performed within device-independent color space such as CIELab space, and there are methods called lightness maintaining compression, minimum colorimetric color difference compression, chroma maintaining compression, and the like. The lightness maintaining compression is suitable for natural images and the like because input lightness and hue are maintained and thus the dimensionality of images can be maintained. The minimum colorimetric color difference compression is suitable for color proofs because a color gamut that is common to input and output devices is reproduced as-is while an uncommon color gamut is associated with the color gamut surface of the output device, and thus input image data can be reproduced in a colorimetrically faithful manner. The chroma maintaining compression is suitable for output of computer graphics, posters or the like because chroma is maintained at a high level.
With the lightness maintaining compression, however, chroma can be reduced significantly depending on the shape of the input color gamut to be compressed, making output images unnatural for viewers. With the chroma maintaining compression, the gradation in the lightness direction is compromised.
In order to solve problems as described above, for example, Japanese Patent Laid-Open No. 2009-219062 proposes a technique in which the degree of similarity between the input color gamut and the output color gamut is detected, based on which the way color gamut compression is performed is changed. FIGS. 8A to 8C show examples in which the input color gamut and the output color gamut are represented in CIE-L*a*b* color space, and the lightness-hue plane is shown with respect to a specific hue. The hue is defined by the ratio between color difference components a* and b*, and a component orthogonal to the lightness axis L* is chroma. Accordingly, the hue plane shows the relationship between lightness L* and chroma c with respect to the specific hue in the L*a*b* color space. As shown in FIG. 8A, if the difference between lightness L1 at the maximum chroma in the input color gamut and lightness L2 at the maximum chroma in the output color gamut is small, it is determined that the input color gamut and the output color gamut are similar in shape, and only chroma is compressed while lightness is maintained. If, on the other hand, the lightness difference between L1 and L2 is large as shown in FIG. 8B, it is determined that they are not similar in shape, and both lightness and chroma are compressed. In this manner, color gamut conversion according to the shapes of input and output color gamuts is performed.
With the conventional technique described in Japanese Patent Laid-Open No. 2009-219062, if the input and output color gamuts are similar in shape, compression is performed while lightness is maintained. In this case, if there is not so much difference between the highest lightness Lw (paper white) and the lightness L2 at the maximum chroma in the output color gamut, in the lightness region above the maximum chroma, there will be a large chroma difference between input and output at the same lightness. Accordingly, if color gamut conversion is performed using a profile that performs mapping from the input color gamut to the output color gamut generally along the chroma axis in order to maintain lightness, as shown in FIG. 8C, a problem arises in that the gradation in the chroma direction is reduced.